This invention relates to a method for the production of ferrochromium with use of a rotary furnace. More particularly, it relates to such method wherein expensive electric power is not used as in the conventional electric furnace method but a carbonaceous reducing agent such as coke or coal is burned in the rotary furnace in the presence of oxygen and raw chromium ores pre-heated or pre-reduced and charged into the rotary furnace are turned under the heat of combustion into ferrochromium by smelting reduction, and to the rotary furnace that may be used for smelting a variety of raw materials including those employed for production of ferrochromium.
Heretofore, in the production of high carbon ferrochromium, the raw materials including raw chromium ores, reducing agent such as coke and fluxes such as lime or silica are charged consecutively into a submerged arc type electric furnace for smelting. By this method, an electric power of 3000 to 3700 KWH is required per each ton of ferrochromium which means a considerable electric power consumption. In consideration of the markedly inferior conversion efficiency from the valuable liquid fuel to electric energy, it is nowadays of vital importance to develop a technique whereby the consumption of a large quantity of electric power may be avoided.
From this standpoint, various methods for the production of ferro-alloy or alloy steels by smelting reduction through use of a rotary furnace, and the rotary furnace used in these methods, have been proposed e.g. in the Japanese Patent Publication Nos. 13043/1965 and 23731/1969. The Japanese Patent Publication No. 13043/1965 shows the method for smelting reduction of the ores in a rotary furnace by using solid reducing agent such as coal and coke. In this method, however, the ores are limited to iron ores and the upper limit of the smelting temperature is set up to 1450.degree. C. for protection of the refractory material. In spite of such limitation, in case of reduction of iron oxides, the refractory material may be attacked by molten iron oxides yielded in the smelting process. Because of this problem, the method of the Publication No. 13043/1965 has not been practiced commercially.
The Japanese Patent Publication No. 23731/1969 shows the method for production of ferro-alloys or alloy steels through smelting reduction making use of a rotary furnace. With this method, the heat source for smelting is limited to oxygen-natural gas or heavy oil or oxygen-carbon monoxide yielded by reducing reactions. Hence it is not possible to use coke or coal which may be readily available at lower costs. It is stated that the materials can be pre-heated or pre-reduced by exhaust gases from a reducing furnace, however, no mention has been made of the method or apparatus for utilizing such exhaust gases.
Moreover, in these known methods, coal, coke or graphite is used as a reducing agent, and the thickness of the layer of reducing agent on the slag surface is set to 30 to 50 mm, leading to a weaker reducing atmosphere and low yield rate. For instance, in case of the production of ferrochromium, Cr yield rate is only about 73 percent. Moreover, in the known methods, the oxygen blow lances are directed upwards or horizontally so that combustion takes place near the refractory material, leading to increased rate of heat radiation or conduction from the refractory material and lower heat efficiency. Moreover, in the case of smelting process where the constant slag temperature should be kept at a constant, the refractory material may be exposed to extremely high temperatures, resulting in increased consumption of the material. Moreover, five metal particles suspending in the discharged reducing agent might not be utilized effectively, resulting to the low yield rate mentioned above.
In addition, in a rotary furnace and as shown in the Japanese Patent Publication No. 23731/1969, oxygen--natural gas or heavy oil or oxygen-carbon monoxide yielded by reducing reactions is blown from a lance towards the refractory ceiling while the furnace which containing charge is rotated about its axis. The charges are smelted by radiation from refractory ceiling heated up by combustion taking place in the furnace.
These rotary furnaces have many practical inconveniences such as mentioned below.
(a) It is not possible to use coal or coke that is available readily at lower costs.
(b) The test results show that the refractories in the furnace may be attacked severely by the slags when the surface temperature thereof has exceeded 1900.degree. C., resulting in acute increase in the consumption of the refractories. With the above known furnace, since the surface temperature of the refractory may reach 2000.degree. C. or higher, consumption of the refractory may increase severely and there arises the necessity for frequently replacing the refractory, thus elevating repair costs and down time for repair.
(c) Smelting may be prolonged because the charges are smelted by radiation heat from the refractories.
(d) The metal yielded may be saturated with carbon used as reducing agents.
In view of these inconveniences, the present invention envisages to provide a rotary furnace in place of the electric arc furnace of the prior art for smelting not only ferrochromium but various other materials and to provide a method for the production of ferrochromium wherein total energy consumption of the process may be reduced by about 25 percent as compared to the electric furnace method through
(a) using coal and coke that are available at reduced costs,
(b) preventing overheating of the refractories in the in the furnace and reducing their consumption,
(c) blowing fuel and oxygen for promoting the smelting of the raw materials, shortening the blowing time and improving the productivity,
(d) decarburnizing the yielded metal for repeated use,
(e) utilizing exhaust gases of the rotary further for preheating of pre-reducing of the raw ores for recovery of waste heat.